A matrix switch apparatus in which a redundancy structure is applied to line interface circuits has been proposed in the U.S. Ser. No. 235,442 filed on Aug. 23, 1988, now abandoned. The matrix switch apparatus comprises a predetermined number of input line interface circuits, a predetermined number of output line interface circuits, and a predetermined number of switch units each positioned between the input and output line interface circuits to be connected thereto. Each of the input and output line interface circuits includes a stand-by interface element and plural interface elements, and each of the switch units includes front, middle and back switches.
In operation, an input signal supplied to an input port of an interface element in one of the input line interface circuits is supplied to one of the front switches in one of the switch units. In the switch unit, the supplied signal is transmitted from the front switch through connected middle and back switches to a predetermined interface element in one of the output line interface circuits. Thus, an output signal is obtained at an output port of the predetermined interface element by the change-over of a connecting path in the front, middle and back switches of the switch unit.
In the matrix switch apparatus, if one of the interface elements in one of the input line interface circuits breaks down, an input port of the broken interface element is switched to be connected to the stand-by interface element, thereby avoiding the suspension of operation. On the other hand, if one of the switch units breaks down, the remaining one of the switch units is selected to operate, thereby also avoiding the suspension of operation. This stand-by mode operation can be carried out in accordance with the redundancy structure including the stand-by interface element in each of the input and output line interface circuits and the provision of the plural switch units. This matrix switch apparatus will be explained in more detail later.
However, the matrix switch apparatus has a disadvantage in that the diagnosis of stand-by ports can not be carried out, where all input ports of the interface elements are occupied for operation, since a connecting path communicating to the stand-by ports is not realized in the switch unit including the front, middle and back switches due to the fact that the scale of each switch unit is small.